Since a high-nitryl resin is excellent in chemical resistance and gas barrier properties it is used as a container material of cosmetics, stationery, chemicals, foods, or the like.
Hitherto, those containers have mainly been formed by extrusion molding or blow molding. However, in recent years, they have also been manufactured by injection molding.
However, since a high-nitryl resin contains a large quantity of nitryl compound, its heat stability is inferior to that of a general resin such as polyolefin or the like. On the other hand, since the melt viscosity is high, if a high nitryl resin is melt kneaded upon molding, heat is easily generated.
In the case of injection molding a resin having such characteristics, generally, a screw of a low compression, a gentle compression, and a deep groove is considered to be suitable. In the case of a high-nitryl resin, in order to prevent the heat generation of the melted resin, a screw of a low compression, a gentle compression, and a deep groove in which a compression ratio is set to about 1.8 to 2.5 and length distribution ratios of the feed section, compression section, and metering section are set to about [1 to 1.5 ]:[1.8 to 2.8]:[1 ]can be preferably used only in the molding of a relatively short time.
However, even in such a screw, there is a problem such that if the molding is continued for a long time, a yellowing occurs in the molded article and a good product cannot be obtained.